In electro-optical sensing systems for scanning an image to provide black and white output representations of parts of the image without intervening shades of grey, such systems will reproduce a relatively faithful image of the original document when the original consists of clean, sharp, black print or typing on a clean, white background. However, if the document scanned is, for example, a fourth generation carbon copy, or contains handwriting in pencil or fine line pen markings, or even has a colored or partially shaded background behind the print, then the legibility of the resulting copy can be impaired. Typically, in systems of the kind referred to above, colored, partially colored or shaded backgrounds result in large black areas on the copy which obliterate the printed information.
A fourth generation carbon copy, when used as an original, will reproduce with very blotchy, illegible print at the output of the scanning equipment. Should some or all of the print on the original provide a low contrast between the print and the background, some or all of the low contrast print will be deleted from the copy.
The problems in such systems manifest themselves in a number of ways, such as by deleting letters or words or portions of either; providing blotchy print with no or few openings in the "e's, o's or b's", dimensional distortion of line widths and position (in the letters in particular), and areas of the reproduced copies will be black or streaked with black where the original page was white.
These problems and others result from the difficulty in making proper decisions as to whether portions of the scanning output signal should be considered as representative of black or white.
For example, the problem can be best explained with reference to FIG. 2 wherein the output video waveform 11 shows a time varying voltage representing the scanner output during a portion of one transverse scan across a page of print. Such a scan is represented by the narrow trace 12 defining the path along which the scan is made. The positive voltage portion of waveform 11 represents white paper background, while the negative spikes or pulses represent black print. A reference voltage typically is established, as at line 10.
Normally, in modern equipment, the decision as to which portions of the waveform represent black or white, is made by a voltage comparator 13 (FIG. 3) in which a given predetermined reference voltage is established, as on lead 13a coupled to potentiometer 13b. As shown in FIG. 3, potentiometer 13b controls comparator 13 whereby (as shown) any voltage level below the predetermined voltage level at the potentiometer setting will be identified as being black and any signal which is more positive will be considered to be white. This arrangement is appropriate for large, black, sharp print on white paper. However, with poor contrast or print which is not sharp and large, the decision as to whether the signal represents black or white becomes difficult. Note the region of waveform within circle 25 and the resultant loss of digital data at 30a of the trace 30. Trace 30 indicates a digital representation of the scanned information.
As shown in FIG. 4, if a reference voltage is set at the level represented by the horizontal line 14 (hereinafter referred to as reference level "A" and a transverse scan of the recorded information is made (as represented by the narrow band 15, defining the path of a scanning trace) the sharply printed letters "NI" will clearly produce appropriate transitions of reference level "A". Accordingly, by dropping beneath level 14, (and using the convention that a voltage below the reference is "black" and above the reference level is "white") the signals derived from scanning the "NI" will be identified as "black". However, the carbon "a" will have no opening in it since waveform 16 provides only a first transition at 17 and a second transition at 18 while the small pulse 19 between transitions 17 and 18 remains undetected since the voltage fails to rise to reference level "A".
If the reference level voltage is adjusted to the line 21 (hereinafter referred to as reference level "B") so as to detect two additional transitions 22, 23 the carbon copy "a" will then be properly identified. The handwritten term scanned along the path 15 cannot be detected by either reference level "A" or "B" and would be completely deleted from any copy made by such a scanner using a comparator system 13. A reference level "C" at 24, however, will reproduce the handwriting but obviously such a reference level will not properly detect the opening in the "a".
While one system is known in which a dynamic reference voltage is developed from the incoming video signal for comparison to a delayed representation of the same video signal, it has been observed that its decision making function provides inconsistent results whereby the lines of a scanned character may be randomly widened or narrowed so as to distort the characters. This system also causes openings to be closed in characters such as "a's, b's, e's" etc.
It has been discovered that by causing the reference voltage to intersect the slopes of the pulses substantially at a predetermined displacement therealong, the foregoing inconsistent printing can be avoided as herein to be disclosed further below.
The system for optically scanning recorded information as disclosed herein provides a reference voltage with respect to and derived from the various pulses representative of black and white portions of information scanned. The reference voltage intersects each pulse substantially at the midpoint or other predetermined proportionate displacement along the positive and negative slopes thereof whereby means for comparing the voltage of one of the signals with respect to the reference voltage for that signal serves to detect crossover of the voltage of said signal with respect to the dynamic reference voltage thereof. Accordingly, the reference voltage as disclosed herein dynamically follows the video waveform and intersects each pulse substantially at the midpoint of each positive and negative slope, (or at another predetermined point therealong, if desired).